This invention relates to circuits for driving vehicle directional and stop indicator lamps, such as those used to drive the left and right turn and stop lamps on a towed vehicle (i.e., trailer) in response to control signals from the towing vehicle. More particularly, the invention relates to a drive circuit of the general type noted above which controls the towed vehicle indicator lamps responsive to corresponding towing vehicle lamp actuation involving different indicator lamp formats, as well as providing towed vehicle brake lamp illumination responsive to actuation of the towed vehicle brakes from the towing vehicle.
In the past, towed vehicle lamp drive circuits have progressed from relatively simple and crude circuits which did no more than connect the towed vehicle directional and stop indicator lamps to the towing vehicle signalling circuit, to interface circuits which allow the directional and stop indicator lamps on the towed vehicle to be connected to towing vehicles having different indicator lamp formats. For example, Bryant U.S. Pat. No. 4,006,453 shows an interface circuit for connecting towed vehicles having dual stop and turn indicator lamps to towing vehicles having either independent brake and turn indicator lamps or dual stop and turn indicator lamps. While the aforementioned state of the art-type systems no doubt have advantages and favorable features compared to past practices, they also have limitations and undesirable attributes. For example, known interface circuits such as Bryant couple the towed vehicle indicator lamps in parallel with the towing vehicle's indicator lamps. The parallel connection interferes with towing vehicle lamp sensing circuitry, which automobile manufacturers often incorporate in vehicles to sense and display the condition of the various lamps on the towing vehicle, as the towed vehicle lamps may draw current even when the towing vehicle lamps do not illuminate. Consequently, when a towed vehicle's indicator lamps are connected in parallel with the towing vehicle's lamps, the towing vehicle sensors will not reliably monitor the condition of lamps on the towing vehicle due to the additional current drawn by the towed vehicle lamps.
Another undesirable characteristic of state of the art-type systems is they do not adequately isolate the towed vehicle indicator lamps from the towing vehicle signal circuits. In some areas, it is required that towed vehicle brake lamps be illuminated whenever the towed vehicle's brakes are actuated, even though this may only involve momentary manual actuation from the towing vehicle for purposes of stabilizing the towed vehicle rather than slowing it. However, due to inadequate isolation of the towing and towed vehicle signal circuits, actuation of the towed vehicle brakes may adversely and undesirably affect operation of the towing vehicle. For example, where the towing and towed vehicle signal circuits are not adequately isolated, such manual actuation of the towed vehicle brakes will cause a brake signal to occur in the towing vehicle signal circuits. When such a signal occurs on the towing vehicle signal circuit, the signal deactivates the towing vehicle cruise control, locks out the towing vehicle torque converter, and injects a rich idle fuel mixture in the towing vehicle engine. Consequently, when the vehicle operator manually actuates the brake controller briefly to stabilize the towed vehicle, both the towing vehicle and towed vehicle will slow down due to the brake signal appearing of the towing vehicle signal line. However, such slowing down on the towing vehicle tends to further destabilize the towed vehicle, making the problem worse or, in any event, making it more difficult for the towing vehicle operator to stop towed vehicle swaying by the usual procedure of manual actuation of the towed vehicle brakes without corresponding braking of the tow vehicle.
Another undesirable result of inadequate isolation of the towed and towing vehicle signalling circuits is the increased load on the towing vehicle turn signal circuit by the addition of the towed vehicle lamps. The additional current drawn by the towed vehicle lamps may damage the signalling circuit on the towing vehicles since they are not usually designed for such high levels of current flow and are not likely to have adequate current-handling ability. Additionally, the increased load on the towing vehicle signaling circuit may cause the turn signals to flash at an undesirably rapid rate.